Articles and process of fabricating



ARTICLES AND PROCESS OF FABRICATING Harry Paul Legg, Columbus, Ohio, assignor, by mesne assignments, to Kaiser Aluminum and Chemical Corporation, Oakland, Calif., a corporation of Delaware No Drawing. Application March 3, 1952-,- Serial No. 274,665

4 Claims. (Cl.- 117-46) The present invention relates to aluminizing. In particular, it relates to a new and novel method or process of preparing the surface of a basis metal to be coated and coating such 'basis metal' with aluminum and its alloys and to the prepared basis metal per se.

In the process of hot dip coating of metals with other metals of lower melting point, difficulty is normally experienced at the moment of entry of the metal to be coated into the coating bath. The difliculty is caused by the heat of the coating bath causing the metal to be coated to oxidize before it becomes fully immersed, whereas it is well known that satisfactory hot dip coatings can only be obtained on metallic surfaces that are scrupulously clean and essentially free of oxide films. In.

coating articles of finite length, that is, fabricated articles, the oxidation is greatest near the last edge of the metal to enter the coating bath.

Although various methods have been devised to overcome this problem, it is difficult tosolve in hot dip aluminum coating of ferrous metals, partly because of the high temperatures involved, and partly because a fully:

satisfactory salt flux for use on aluminum b'aths' is not available, while protective gaseous fluxes or covers arediificult to maintain.

Hence, it is a primary ob ect of the present lIlVfil'le tion to provide a new and novel method of hot dip coating basis metals-with aluminum and its alloys wherein oxidation of the basis metal before it becomes fully immersed in the moltenbath is avoided.

It is another important object of the present invention to provide a new and novel. method of aluminizing,

characterized by elimination of individual or separately applied fluxes or protective atmospheres over the surface of the aluminum containing bath.

It is yet another object of the present invention to provide a new and novel method of alumlmzing wherem the resultant coating is continuous.

A further object of the present invention is to provide a new and novel method of aluminizing in which the basis metal is .protected from atmospheric attack up to and during the moment of entry into the molten aluminum or aluminum alloy bath.

A still further object of the present invention is to provide a new and novel basis metal having a coating characterized by the fact that such' coating affords pro tection to the cleaned basis metal against atmospheric attack prior to and during the act of immersion anddecomposes by the bath action to produce a reducing atmosphere.

These and other objects and advantages of the present invention will become more apparent from the following detailed description and example.

It has now been found that complete and continuous aluminum and aluminum alloy coatings on basis metals can be readily obtained without the necessity of employing salt or gaseousfluxes or atmospheres over the meltby merely cleaning the basis metal and priorto dipping,

providing it with a continuous, impervious and intimatewax coating decomposable by thebath.

In practice, the article or basis metal to be coated; 1s first cleaned, etched with acid if-need' be, or otherwlse prepared for coating. This article or material is then brought in a clean and dry condition to a bath of the wax selected, which is readied in advance in a container of suitable size, maintained at atemperature above the melting point of the particular wax or wax. m xture selected. The article or material is then dipped into the "ice molten wax; The'timemffimmersion in the molten: wax: may bewidely variedand is.notedeemedcritical, it being:

necessary only touprovide a continuous wax coatingom the basis metal.

The article :orxmaterial. itoi beidipped, on removal from:

the waxlbathjis theniready.forzimmersionrinto theemetallie.

tially impervious to moisture and air so theiwax coated;

part may be stored for,considerable-periods; of. time. without oxidation of thecleanedsurfaceof? the basis; metal.

At the moment of'immersion offlthewwax coated part into the molten metal,coatingbath;withouta protective flux or atmosphere cover,.the wax, suddenly, brought in. contact with hot liquidmetal bursts intoflame, providing, a smoky reducing atmosphere about the metal; Pro: tected by this atmosphere, thesolid metal: is immersedinto the coatinghbatn without; oxidation and on with.- drawal. and cooling,results.-,in..a completely. and 3COI'ltiI1llously coated article.

Another. advantage in the practice of. this invention; thereupon becomes evident,.particular ly in molten aluminum baths. Molten aluminum in contact withair ac-; quires a tough coherent oxide film; that: resists: parting, under the article or material-beingdipped. The filmmay fold'about. the-article entering the bath, andbe carried underthe bath surface, often momentarily, entrapping air between the filmand the surface of the basismetal. This entrapped air, hCEltBdbYjthti molten aluminumbath, very rapidly oxidizes the metal with which it is heldin; contact. Such oxidized surface cannot be. satisfactorily coated with aluminum. However, if'the dipped metal has previously been coated with wax, some small part ofthis wax is carried under thebath surface. Under the surface of the molten aluminum, the wax, or decomposi-- tion productof the-wax, carried under the surface erupts violently but harmlessly into a comparatively large volume of reducing gas This large volume of gasis quickly-ex: pelled from the molten aluminum bath, sweeping the surface of the immersed metal and allowingthe. molten aluminum to come in contact with a clean. non-oxidized surface ofthatmetal'; Still another advantage found in the practice of. this. invention results when new andIpreviously untried shapes. of fabricated articles'are to be clipped in aluminum. Often. times determination of the exact angle of suspension for satisfactory rapid elimination. of'air. from various-res cessesis difiic'ultl and may require several trials to find a satisfactory solution- It. has been. found that the use of wax coatings on such parts-decreases the number 'of trials required to determine an'a'dvan'tageo'us point of suspension" from which. to support. such articles at the moment of immersionqinto theliquidsrnetal'coatingbath;

The wax used in practice of the present invention should be one which. readily'decomposesior is reduced, etc., at the bath operating temperatureto provide the necessary reducing. gas. At room or. normalIfoundry temperature, it. preferably should form a solid, adherent, continuous coating on the 'cleanedba'sis metal which is impervious to attack'by atmospheric gases. The thickness of thecoatingis not'criticalfbut obviously should be of sufiicient thickness to withstand atmospheric attack and, if the coated basis material is not to be immediately hot dipped, to' withstand handling,.'storage-, etc. The coating need not be frozen although this is obviously desirable if the article be stored, but it can be somewhat plasticor viscous, either by not having. completely solidified' from the melting temperature or from incomplete evaporation of the solvent if used,, or by using a lower melting point wax or wax? composition if the wax coated basis metal is to be shortly or immediately aluminized, as forexample, inthe coating of strip or wire. A wax Patented Jan. 4, 1955 a or mixture of waxes found suitable for the purposes of the present invention is selected from the following group of petroleum refinery products; slop wax, slack wax, or derivatives of one of these such as microcrystalline wax, or paraffin, or a solution of one or more of these waxes in pale oil or other volatile solvent. It is preferred to use low ash, petroleum base, parafiin or naphthenic waxes or mixtures thereof which have melting points of from about 100 to 180 F., flash points of from about 475 to 525 F., and spontaneous ignition points of from 550 to 625 F. As is well known in the art, all of the foregoing substances are hydrocarbon type waxes of mineral origin.

Basis metals treated according to the present invention are those which will form an interfacial layer when dipped in an aluminum or aluminum alloy melt and, thus, will be provided with an adherent coating to protect it from corrosion, friction, and other attack. This invention is also applicable to basis metals which are provided with merely decorative aluminum or aluminum alloy coatings. While the present invention has particular reference to ferrous metals and their alloys that is, plain carbon steels and alloy steels such as those containing molybdenum, tungsten, vanadium and stainless steels of chromium and chromium and nickel, it is obvious that other metals can be very desirably coated with aluminum according to the present invention and therefore the present process will also cover basis metals such as chromium, cobalt, molybdenum, nickel, tantalum, titanium, tungsten, vanadium, zirconium and their alloys. Fabricated, partly fabricated, or unfabricated basis metals, for example, sheet, strip and wire can also be used. The present invention is particularly applicable to basis metals which are fabricated of thin gauge metal, for example, fabricated stove parts of thin sheet steel. As stated above, the basis metals to which the invention is particularly applicable are those which oxidize under the heat of the molten metal coating bath prior to immersion.

Prior to hot dipping, the basis metal is provided with a surface receptive to the hot coating metal. Such may be accomplished by a combination of any of several methods well known in the art which leave a chemically clean, etched surface, that is a surface free of oil, grease, dirt, oxides and slightly etched. The surface of the basis metal should not be polished which tends to prevent metal coating and should not be so porous or rough that wax decomposition products are entrapped therein to cause pitted or raised areas in the final aluminum or aluminum alloy coating. It may be degreased or deoiled by treatment with a hydrocarbon solvent, for example, carbon tetrachloride or vapor degreased using trichloroethylene. After degreasing, the basis metal may also be washed with a strong soap or alkali solution. Finally, the basis metal may be pickled in an acid solution such as a 50-50 concentrated HCl-HzO solution. In some cases, the surface need only be oxidized and then pickled. Simple sand blasting is sufiicient preparation in most cases.

Aluminum or aluminum alloy baths used to coat basis metals according to the method of the present invention can be pure aluminum melts considered as having 99+% or more aluminum, balance adventitious materials or aluminum base alloy melts capable of satisfactorily coating basis metals and containing as little as above about 50% aluminum and the balance a metal which advantageously improves, modifies, etc., the aluminum and which can be at least one metal selected from the group consisting of beryllium, boron, cerium, chromium, copper, cobalt, magnesium, molybdenum, silicon, titanium, tungsten, vanadium, zinc, zirconium and so forth plus the usual impurities. The foregoing shall constitute the definition of the term "aluminum base alloy as used throughout the specification and in the appended claims.

The basis metal whether fabricated or unfabricated can be coated by automatic or manual processes. When automatically dipping articles, instead of using hand or manual dipping methods, the articles can be mounted on jigs or other suitable apparatus and by conveying means lowered into the melt, held there momentarily, raised to permit drainage, and then carried onto the next fabricating step. When continuous methods are used for coating strip or wire, the strip, for example, is introduced into the bath and carried around means such as a pulley immersed some distance in the bath. When manually dipping articles, it is customary to skimthe bath before dipping and removing the article in order to avoid staining and entrapment of occlusions. In continuous methods, skimming is performed usually at the beginning of operations. Further, although unnecessary, it may be desirable for some operations to provide with the present process a cover over the molten bath such as a molten salt, charcoal, or other relatively inert material to prevent undue heat loss and oxidation of the melt. In this case, an entry tube is desirable to prevent the cover from attaching itself to the wax coated article, etc., or from destroying the wax coating. The cover is usually wiped away locally in withdrawing an object that is being hot dipped to avoid adherence of objectionable particles on the finished product.

The time of immersion will vary from a few seconds when coating strip to possibly several minutes where massive fabricated parts are used. The time should be sufiicient so that the article will arrive at the coating temperature and form the necessary bond or interfacial layer by diffusion. The bath operating temperature will be normally from about 1150" F. to about 1375 F. depending on the particular bath composition employed although it is realized that the melting point of the coating composition may be different as, for example, pure aluminum melts at about 1220 F., a 12% silicon balance aluminum alloy melts at about 1070 F., and other aluminum alloys used in hot dip coating melt at from 1070 to 1220 F. The times and temperatures for coating are accordingly selected to produce optimum results and are obvious to those skilled in the art. However, it will be appreciated that they should be conducted so as to avoid a large interface layer yet be sufficient to provide an even and smooth coating, unless only the interface or compound layer is desired, as, for example, when producing articles having scale resistance at high temperature.

The following example will serve to illustrate the invention with more particularity to those skilled in the art:

Sheets, strips and fabricated articles of plain carbon SAE 1040 steel were cleaned of grease and oil films by rapid oxidation at an elevated temperature. After cooling, they were immersed in a pickling solution of a 50-50 mixture of concentrated HCl and water for two minutes. Rinsing and drying, followed the pickling step, after which the various articles were immersed in a molten 13% silicon, balance aluminum alloy bath at a temperature of 1200 F. A gas fired 11 inch pot was used for dipping. The results obtained are shown below:

Irnrnerg Description of item Notes (ruins) 1 4" x6 3 to strip across top of sheet not coated.

2 1" x 685e. strip 3 Prceoated with Cu; entire strip coated.

3 do 3 Preeoated top half with On;

top inch not coated.

4 3 x }6 U. iron cold- 8 Precoated with On; many rolled. raised areas; one large area uncovered.

5- 4 x6 sheet 8 to strip across top not coated.

6 do 8 Razor edge ground across top;

completely coated.

7 do 8 Not sharpened; not coated at top of sheet.

8. Section of burner 8 Ground edge at top; not coated at top inch.

9- Gorrugatedheat trans- 8 Large spots under top ridge fer section. not coated.

10...- 4" x 6 sheet 8 Ground edge at top; not coated at top.

It, thus, is seen that articles which are merely cleaned and pickled before immersion in molten aluminum are not always completely coated, that the completely coated articles have certain raised areas or blisters or may be pitted, and that of a group of articles of the same type of steel, are not coated. An additional run was made on the same type of uncoated steel sheet, strip and fabricated articles, as shown in the above table. They were cleaned and pickled in the same manner. After rinsing and air drying, the samples were dipped in a melted F.) slack wax composition. The articles were withdrawn, the wax allowed to solidify and they were then immersed in the same aluminum alloy bath.

On withdrawing and cooling, they were found to be completely covered with an adherent, complete, continuous, and even aluminum alloy coating.

In summary, the present invention teaches that complete and continuous aluminum and aluminum alloy coatings can be readily produced on basis metals by merely providing the cleaned basis metal with a coating of a decomposable wax which provides protection against atmospheric attack prior to and during immersion in the melt. The present process also eliminates the equipment and material necessary to provide a salt flux or protective blanket of reducing or inert atmospheres over the melt since the proper atmosphere is actually provided precisely at the point where it is needed by the decomposing or burning wax. Moreover, the process herein disclosed permits the complete coating of metals of thin cross section, such as sheets, strips or wires, which are subject to rapid oxidation due to high heat conductivity. Again, the cleaned and wax coated basis metals shown herein can be stored indefinitely until they are ready for use thereby eliminating customary bright annealing or fluxing steps customarily required just prior to dipping or the need for immediately immersing the cleaned and pickled basis metal in the aluminum containing bath. Finally, the technique disclosed herein of wax coating a cleaned basis metal prior to hot dipping lends itself readily to commercialization as it requires no complicated equipment nor process control and the savings in the use of fluxes and protective atmospheres with attendant equipment and required control and in the elimination of incompletely coated basis metals more than offsets the cost of the wax.

Having thus described the present invention, what is claimed as new and novel and is desired to be secured by Letters Patent is:

1. The method of producing on ferrous metal a continuous coating of a metal selected from the group con sisting of aluminum and aluminum base alloys, which comprises cleaning the ferrous metal, coating said cleaned ferrous metal with a continuous coating of a hydrocarbontype wax of mineral origin adapted to burn at the melting temperature of said coating metal to produce a reducing atmosphere, and then immersing the wax-coated ferrous metal in a bath of molten metal of said group having a temperature suflicient to burn said wax thereby providing a reducing atmosphere to prevent oxidation of said ferrous metal.

2. A method according to claim 1 wherein the Wax has a melting point from about to F., a flash point of from about 475 to 525 F., and a spontaneous ignition point of from about 550 to 625 F.

3. A method of producing a continuous coating of aluminum and aluminum base alloys on a dissimilar metal with a molten bath of the coating metal, wherein the dissimilar metal is oxidized by the heat of the molten metal coating bath prior to immersion, comprising the steps of cleaning said dissimilar metal, coating said metal With a continuous coating of a hydrocarbon-type wax of mineral origin adapted to burn at the temperature of the molten metal coating bath and to produce a reducing atmosphere, immersing said Wax coated metal in the molten metal coating bath having a temperature suflicient to burn said wax thereby providing a reducing atmosphere to prevent oxidation of said dissimilar metal.

4. A method according to claim 3 wherein the wax has a melting point from about 100 to 180 F., a flash point of from about 475 to 525 F., and a spontaneous ignition point of from about 550 to 625 F.

References Cited in the file of this patent UNITED STATES PATENTS 842,758 Brown Jan. 29, 1907 1,098,368 Caflall June 2, 1914 1,339,710 Page May 11, 1920 FOREIGN PATENTS 425,086 Great Britain of 1935 

1. THE METHOD OF PRODUCING ON FERROUS METAL A CONTINUOUS COATING OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM BASE ALLOYS, WHICH COMPRISES CLEANING THE FERROUS METAL, COATING SAID CLEANED FERROUS METAL WITH A CONTINUOUS COATING OF A HYDROCARBONTYPE WAX OF MINERAL ORIGIN ADAPTED TO BURN AT THE MELTING TEMPERATURE OF SAID COATING METAL TO PRODUCE A REDUCING ATMOSPHERE, AND THEN IMMERSING THE WAX-COATED FERROUS METAL IN A BATH OF MOLTEN METAL OF SAID GROUP HAVING A TEMPERATURE SUFFICIENT TO BURN SAID WAX THEREBY PROVIDING A REDUCING ATMOSPHERE TO PREVENT OXIDATION OF SAID FERROUS METAL. 